The so-called high density ethylene polymers having annealed densities of at least 0.95 are customarily prepared by polymerizing ethylene in the presence of solid catalysts of the so-called Ziegler-type, which are reaction products of a salt of a transition metal and a metal alkyl, typically an aluminum alkyl. Many of the newer Ziegler-type catalysts product ethylene polymers of unusually high molecular weight in that they have intrinsic viscosities of at least 3. Because of their high molecular weight and concomitantly high melt viscosities, such ethylene polymers are difficult to fabricate by many common plastic fabricating techniques such as extrusion. It would be desirable to have available to the art techniques for modifying the processing characteristics of such high molecular weight ethylene polymers.